Such a metal-clad substation is constituted by a plurality of bays which are connected in parallel by a set of feeder busbars, each including in series with the circuit breaker a busbar disconnector (or selector switch disconnector) and an outgoing feeder. Each piece of gear in a bay is enclosed in a gastight enclosure filled with a dielectric gas under pressure for the purpose of maintaining a potential difference with a conductor that passes along the enclosure. A bay is thus constituted by a plurality of compartments constituted by the enclosures of various different pieces of electrical gear.
An electric arc which occurs between the enclosure and the conductor of a compartment of the bay is referred to as an internal arc. In conventional manner, a substation protection system is provided for detecting such a fault by measuring the current passing through the substation. However, the protection system does not make it possible to locate the internal arc, so it is not possible to identify the bay of the substation or the compartment within said bay in which the internal arc has occurred.
The compartments of each bay are provided with respective pressure sensors designed to measure the pressure of the dielectric gas to be found inside the various enclosures. If an internal arc occurs in a compartment, then the pressure detector detects an increase in pressure, thereby enabling the compartment to be identified.
Identifying the faulty compartment by an increase in pressure presents no difficulty with a disconnector or with an outgoing feeder from a bay.
In contrast, a problem of discrimination arises with a circuit breaker. When the protection system detects a current fault in the electrical substation, it issues a disengagement order to the circuit breakers which then open on receiving the order. On opening, each circuit breaker strikes a respective short circuit arc known as a circuit-breaking arc, thereby increasing the pressure of its dielectric gas.
To determine whether an internal arc has occurred in a particular circuit breaker of one of the bays of a substation, it is essential to be able to identify a circuit-breaking arc which inevitably follows the disengagement order issued by the protection system.
An apparently-satisfactory solution would base identification on the amplitude of the increases in pressure due to an internal arc as compared with those due to a circuit-breaking arc. Nevertheless, that solution is not applicable under all circumstances, in particular in the event of internal arcs that are small, since the increase in pressure that they cause is of the same order of magnitude as the increase in pressure caused by breaking a high short-circuit current.